Technology for integrating a metal and a synthetic resin is needed in a wide range of industrial fields such as automobiles, consumer electrical equipments, industrial machineries and other such part manufacturing industries and various adhesives have been developed for this purpose. Some of these adhesives are very excellent. For example, adhesives that exhibit their function at normal temperature or under heating are used to join and integrate metals and synthetic resins, so this method is at present the standard joining technique.
On the other hand, research has been conducted into a more rational joining method that does not entail the use of any adhesive. An example of this is a method in which a high-strength engineering plastic resin is integrated with a light metal such as magnesium, aluminum, an alloy thereof or an iron ally such as stainless steel without any adhesive. For example, the inventors have proposed a method in which molten resin is injected into a metallic mold into which a metal part is preliminarily inserted thereby forming a resin part and at the same time a molded part is joined to the metal part (such method being hereinafter abbreviated as “injection joining”).
The proposed method concerns a manufacturing technique with which a polybutylene terephthalate resin (hereinafter referred to as “PBT”) or polyphenylene sulfide resin (hereinafter referred to as “PPS”) is injected and joined to an aluminum alloy (see Japanese Patent Application Laid-Open 2004-216425: Patent Document 1, for example). There has also been disclosed a joining technique in which relatively large holes of 25 nm or greater are formed in an anodized film of aluminum material and synthetic resin protrudes into the holes, thus bonding being made (see WO2004-055248: Patent Document 2, for example).
The principle of injection joining proposed in Patent Document 1 is as follows. Aluminum alloy is immersed in dilute aqueous solution of water-soluble amine compound and the aluminum alloy is finely etched by the aqueous solution of weak basicity. It was found that adsorption of the amine compound molecules to the surface of the aluminum alloy occurs at the same time in this immersion. After undergoing this treatment, the aluminum alloy is inserted into the metallic mold for injection and a molten thermoplastic resin is injected under high pressure.
Heat is then generated through contact between the thermoplastic resin and the amine compound molecules adsorbed to the surface of the aluminum alloy. Substantially at the same time as this heat generation, the thermoplastic resin is quenched by contact with the aluminum alloy, which is kept at low temperature of the mold, and therefore the resin, which was apt to solidify as well as crystallize, is caused to make its solidification retarded and protrude into the extremely fine concave portion on the surface of the aluminum alloy. Consequently, the aluminum alloy and the thermoplastic resin are securely joined (affixed) without the resin coming loose from the surface of the aluminum alloy. That is, when exothermic reaction occurs, secure injection joining can be obtained. It has been actually confirmed that PBT or PPS, which undergoes an exothermic reaction with an amine compound, can be joined by injection joining to this aluminum alloy. With another well known technique, chemical etching is performed preliminarily, then a metal part is inserted into the metallic mold of the injection molding apparatus and injection molding is performed using a thermoplastic resin material (see Japanese Patent Application Laid-open 2001-225352:Patent Document 3, for example).
However, while joining based on the principle mentioned above does have an extremely good effect with aluminum alloys and the like, the same effect is not necessarily obtained in injection joining to metals other than aluminum alloys. There has therefore been a need for the development of a new joining technique. The inventors discovered a novel technique while developing and improving the injection joining of hard resins to aluminum alloys. Specifically, conditions were discovered under which injection joining is possible without chemical adsorption of an amine compound to the surface of a metal part or, in other words, without help of an extra exothermic reaction or any special chemical reaction.
At least two conditions are necessary for this. The first condition is that a hard resin of high crystallinity be used, that is, that PPS, PBT or an aromatic polyamide be used, while this condition alone does not provide the desired result and more practical joining will be obtained by making these a composition that is further improved and suited to injection joining. The other condition is that the metal part that is inserted into the mold have a strong and hard surface layer.
For example, when a shaped magnesium alloy is used as the base material, this alloy is subjected to a chemical conversion treatment or an electrolytic oxidation treatment to produce a surface of metal oxide, metal carbonate or metal phosphate which creates a surface covered with a hard ceramic substance, since corrosion resistance is low with a magnesium alloy that is only covered with a natural oxidation layer. The above-mentioned conditions can be met with magnesium alloy parts having these surface layers.
Theoretically, the following applies if we consider a case in which a shaped and surface treated magnesium alloy is inserted into a metallic mold for injection molding. Since the metallic mold and the inserted shaped magnesium alloy are kept at a temperature that is at least 100° C. below the melting point of the resin to be injected, it is very likely that the injected resin will be quenched as soon as it flows into the channel inside the metallic mold and the temperature will drop below its melting point upon coming into close proximity with the magnesium metal part.
When crystalline resin is suddenly cooled from a molten state to a temperature below its melting point, it does not crystallize and solidify instantly, regardless of the type of crystalline resin and there is a certain time, albeit short, during which the resin remains in a molten state below its melting point, that is, in a super-cooled state. If the concave portions of the shaped magnesium alloy have a relatively large diameter of about 100 nm, there will be enough time for the resin to enter the concave portions within the limited time in which microcrystallization occurs from super-cooling. Even if the numerical density of the macromolecular crystal groups thus produced are still low, the resin will still be able to enter the concave portions as long as they are large enough. This is because the size of microcrystals, more specifically microcrystals having a shape formed when a change from irregularly moving molecular chains into molecular chains with some kind of ordered state occurred, is considered to be from just a few nanometers to 10 nm, assuming a molecular model.
Therefore, it is not allowed necessarily to say that microcrystals can easily infiltrate ultrafine concave portions having a diameter of 20 to 30 nm, but it is possible to determine that they can infiltrate if the concave portions are about 100 nm in size. However, since countless microcrystals are produced simultaneously, there is a sharp rise in the viscosity of the resin flow at distal ends of the injected resin and at positions where it touches the face of the metallic mold. Therefore, if the concave portions are about 100 nm in size, the resin may not be able to infiltrate all the way inside of the concave portions, but will crystallize and solidify after infiltrating to a substantial extent into the interior, so a considerable joining strength is obtained. Here, if the surface of the shaped magnesium alloy is of a hard and strong surface layer comprising an amorphous layer or a ceramic microcrystal group such as a metal oxide, there will be better hooking between the shaped magnesium alloy and the resin, that is, the joining strength will be higher.
In addition to the two conditions mentioned above and related to surface treatment of the shaped material, the present invention adds an improvement of the resin composite to be injected. This relationship will be described below. When injection molding has been performed, the resin composite is quenched from a molten state to a temperature below the melting point and, if the resin composite has a lowered crystallization speed, the joining strength will be higher. This is a condition for a resin composition suitable for injection joining.
On the basis of this, the inventors discovered and proposed that a hard crystalline resin can be joined to a shaped magnesium alloy by injection joining so as to obtain a high joining strength by chemically etching a shaped magnesium alloy and then subjecting it to a chemical conversion treatment or other such surface treatment to make the surface layer a hard ceramic (Japanese Patent Application 2006-272832). Specifically, if there is suitable texturing, at least for all metals and metal alloys, there is a possibility of injection joining by using PBT or PPS that has been improved for injection joining.
What has been disclosed as prior art will now be described. Patent Document 3 discloses a method in which chemically etched copper wires are inserted into an metallic mold for injection, PPS or the like is injected and a battery cover with lead wires attached is produced in a configuration in which a plurality of copper wires stick out from the middle portion of a PPS disc. It is stated that a feature of the art is that because of surface texturing (roughness) of the copper wires by chemical etching, no gas leaks through the lead wire portions even if the internal pressure of the battery should rise.
The technique disclosed in Patent Document 3 is not an injection joining technique but is instead an injection molding technique that utilizes the relationship between the coefficients of linear expansion of metal and molding shrinkage of resin. In the case where resin is injected into the peripheral portion of a sticking out, rod-shaped metal substance in the mold and thereafter the molded article is taken out of the mold and allowed to be cooled, the metal rod will be in a state tightly fastened by the molded resin part. The reason is that the coefficient of linear expansion of metal is at most 1.7 to 2.5×105° C.−1 for aluminum alloy, magnesium alloy, copper or copper alloy and even when the product is taken out of the mold and cooled to room temperature, the shrinkage in an order of the coefficient of linear expansion×100° C. is only about 0.2 to 0.3%.
On the other hand, molding shrinkage for resin is about 1% for PPS and 0.5% for PPS containing glass fiber and even for resin with an increased filler content the resin part always shrinks more than the metal part after injection molding. Therefore, if a molded article with a metal part in the middle and a sticking out resin part is produced by injection molding with an insert, an integrated product in which the metal part is unlikely to come loose can be manufactured because of the tightening effect produced by the molding shrinkage of the resin part. Also, the art disclosed in Patent Document 3 does not involve affixing of a resin to a metal but instead is a joining technique for reducing the leakage of internal gas through the joint faces of the two components. That is, it is not an art premised on the affixing of the two.
This method for manufacturing an integrated metal and resin of the fastened type is known conventionally and a similar molded article is the handle of a kerosene heater. A thick iron wire with a diameter of about 2 mm is inserted into a metallic mold for injection molding and heat-resistant resin or the like is injected. Serrations (knurling) is formed on the wire to keep the resin in place. The art disclosed in Patent Document 3 has a feature such that the texturing is made more efficient by changing from physical working to chemical working, which also makes the texturing somewhat finer, and that a resin which is hard and crystalline is used for raising gripping effect.
There is no need whatsoever for the resin to be enveloped with the present invention. When two flat shaped plates are joined together, it needs a tremendous force to break them apart. A major feature of the technique of the present invention for increasing joining strength is the use of a crystalline resin composition with high hardness that crystallizes and solidifies over a long super-cooling period during quenching.
For the joined state of a metal and a thermoplastic resin to be maintained stably over an extended period, it is actually necessary for the coefficients of linear expansion of the two materials to be of close values. The coefficient of linear expansion of thermoplastic resin composition can be lowered considerably by adding a large quantity of filler, namely, glass fiber, carbon fiber or another such reinforcing fiber but the limit for this is (2 to 3)×10−5° C.−1. Types of metal that are close to this value at an approximate room temperature include aluminum, magnesium, copper and silver.